In a typical cellular telephone network, a user communicates using a handset. The handset is commonly referred to as a Mobile Station (MS). The mobile station is in bidirectional radio communication with a Base Station (BS), and the base station is a part of a network of base stations. The mobile station of a user may initially be within a communication region of a first base station and may be communicating with a first base station. Then, as the user moves, the mobile station moves out of range of the first base station and enters communication range of a second base station. The mobile station begins to communicate with the second base station. In this way, the user can remain in communication with the network of base stations as the user moves from base station communication region to base station communication region. Communication from the base station to the mobile station may be referred to as DownLink (DL) communication, whereas communication from the mobile station to the base station may be referred to as UpLink (UL) communication. Such a network of base stations and mobile stations is not limited to use in cellular telephone communications, but rather sees general utility in many different applications and in accordance with many different protocols and standards for many purposes, including for general high speed data communication purposes.
In some such networks, communications between base stations and mobile stations is subject to interference and loss of the data being communicated. A technique referred to as Hybrid Automatic Repeat ReQuest (HARQ) is therefore sometimes employed. In a standard Automatic Repeat ReQuest (ARQ) method, error detection bits are added to data to be transmitted. In Hybrid ARQ, error correction bits are also added. When the receiver receives a transmission, the receiver uses the error detection bits to determine if data has been lost. If it has, then the receiver may be able to use the error correction bits to recover (decode) the lost data. If the receiver is not able to recover the lost data using the error correction bits, then the receiver may use a second transmission of additional data (including more error correction information) to recover the data. Error correction can be performed by combining (soft combining) information from the initial transmission with additional information from one or more subsequent retransmissions. There are several types of HARQ systems. In some, a retransmission includes error correction and detection information, but no data. In other types of HARQ systems, a retransmission includes data as well as error detection and correction information.
FIG. 1 (Prior Art) is a simplified diagram that illustrates a HARQ technique. The upper row 1 of blocks includes data bits 2 and channel encoding bits 3. The channel encoding bits 3 in this example include error detection and error correction information. If data 2 is properly communicated from the transmitter to the receiver in a first transmission 4, then few of the channel encoding bits are required to recover the data at the receiver. Accordingly, in a first transmission 4, the data bits and only some of the channel encoding bits are communicated. If the data can be property recovered (decoded), then no more transmissions are necessary. If, however, the first transmission 4 is corrupted to such an extent that the data cannot be decoded at the receiver, then a second transmission 5 is attempted. In this technique, only some of the channel encoding bits that were not sent in the first transmission are sent in this second transmission because not all of the channel encoding bits are necessary for the receiver to decode the data. If the receiver receives second transmission 5 and can decode the data, then no more retransmissions are required. If, however, the receiver still cannot decode the data, then a third transmission 6 is performed. Similarly, a fourth transmission 7 may be performed. Each successive transmission communicates more and more of the channel encoding information and hopefully at some point in the retransmission sequence the receiver will be able to decode and recover the data without errors.
FIG. 2 (Prior Art) is a diagram that illustrates a structure of a frame in accordance with a communication protocol employing a HARQ technique. The frame structure of FIG. 2 is not an actual frame structure, but rather is a simplified illustration presented here for instructional purposes. The frame includes a downlink sub-frame 8 and an uplink sub-frame 9. The downlink sub-frame 8 is communicated from the base station to one or more mobile stations in communication range of the base station. The uplink sub-frame 9 is communicated from a mobile station back to the base station. There may be many mobile stations in communication range of the base station. In order for the mobile stations not to transmit uplink sub-frames at the same time and to interfere with each other, the base station includes a UL_MAP 10 in the downlink sub-frame. The UL-MAP 10 indicates to the mobile stations which particular mobile station is to communicate an uplink sub-frame, and when that mobile station is to transmit this sub-frame. As is known in the art, the units of the vertical axis represent various sub-channels that may be used for communication. The horizontal axis represents time extending from left to right. The downlink sub-frame is therefore followed in time by the uplink sub-frame. In one HARQ example, the base station communicates the first transmission 4 of FIG. 1. The DL-MAP 11 of the first transmission includes numerous information elements (IEs), as well as numerous bursts of data. Each burst includes one or more sub-bursts. In the diagram of FIG. 2, BURST#1 includes a first sub-burst (sub-burst#1). Sub-burst#1 includes a cyclic redundancy check (CRC) code 12. CRC code 12 is an error detection code. The receiver receives this first transmission 4, and uses the DL-MAP to determine where in the following part of the DL sub-frame the sub-burst#1 is found. The receiver then uses CRC code 12 to check the integrity of sub-burst#1. If the CRC check succeeds, then the data of the sub-burst#1 is determined to have been successfully communicated and no retransmission of the sub-burst#1 is necessary. The receiver responds by returning an ACKnowledgement (ACK) to the transmitter. The receiver communicates this ACK at a time in a following UL sub-frame. The time and place in the UL sub-frame was previously specified to the receiver by the transmitter in an IE of the UL-MAP. If, on the other hand, the receiver cannot properly decode the data as indicated by the CRC check failing, then the receiver returns a Negative ACKnowledgement (NACK) at the specified time and place in the UL sub-frame. In the example of FIG. 2, the ACK or NACK is returned in ACK channel 13 in uplink sub-frame 9. The diagram of FIG. 2 is simplified. It is to be understood that the actual UL sub-frame containing the ACK or NACK may be transmitted several frames following the frame that contained the data (sub-burst#1 in this example).
If the transmitter (Base Station) receives an ACK, then the transmitter does not retransmit the sub-burst because the data of the sub-burst was properly decoded. If, however, the transmitter receives a NACK, then the transmitter retransmits the sub-burst in the form of second transmission 5 to the receiver. This second retransmission 5, of course, occurs in a subsequent DL sub-frame. If the receiver then returns an ACK, the communication is completed. If the receiver returns a NACK, then the transmitter sends another retransmission 6 of the sub-burst. In the example of FIGS. 1 and 2, the transmitter may attempt three retransmissions (four transmissions total) before giving up on communicating the sub-burst data.
FIG. 3 (Prior Art) is a signal diagram of a problem referred to as a “twice-toggling” problem. In a signal diagram such as FIG. 3, time proceeds from top to bottom in the diagram. The base station uses a sequence number (AI_SN) communicated with a sub-burst to indicate whether the sub-burst is a retransmission of a previously transmitted sub-burst or is a transmission of a new sub-burst. If the AI_SN sequence number is the same as in a previous transmission, then the receiver considers the transmission to a retransmission of the previous sub-burst. If, however, the AI_SN sequence number is detected to have toggled, then the receiver considers the transmission to be a transmission of another sub-burst.
In the example of FIG. 3, the base station (BS) attempts a first transmission HARQ SUB_BURST#1 14 of sub-burst#1 to a mobile station (MS). The CRC check 15 fails, so the mobile stations returns a NACK 16. The BS receives the NACK, and responds by retransmitting the sub-burst#1 17. The transmission is a retransmission, so the AI_SN of the retransmission is a “1” and is the same as the AI_SN value of the initial transmission 14. The MS receives retransmission 17 and still cannot properly decode the data. The second CRC check 18 fails as indicated by the label (CRC NOK). Four transmissions of sub-burst#1 are attempted (the initial transmission, and three retransmissions), until the BS gives up at time 19.
After giving up at time 19, the BS attempts transmitting a second sub-burst, sub-burst#2 20. Because transmission 20 is a transmission of a sub-burst other than the first sub-burst#1, the sequence number AI_SN is toggled to be “0”. This transmission 20 is, however, in this example not received by the mobile station. The star symbol 21 indicates that the transmission is not received. After a certain amount of time, when the BS does not receive either an ACK or a NACK, the BS attempts to send the sub-burst#2 once more in a retransmission 22. Retransmission 22 is also not received as indicated by star 23. After four attempts, the base station gives up attempting to send the second sub-burst#2.
The BS then attempts communicating a third sub-burst, sub-burst#3. Because this transmission 24 is a transmission of a new sub-burst, the AI_SN sequence number is again toggled to be a “1”. Transmission 24 is received by the MS in this example. The mobile station erroneously attempts to combine transmission 24 with the collected information from the previously received sub-burst#1 transmissions because the AI_SN sequence number indicates that the sub-burst of transmission 24 is the same sub-burst as the sub-burst of the previously received sub-burst#1 transmissions. This “soft combining” error occurs because the MS did not receive any of the four attempted sub-burst#2 communications that had the AI_SN value of “0”.
FIG. 4 (Prior Art) is a simplified diagram of a problem referred to as a “buffer overflow” problem. The BS initially transmits a sub-burst, sub-burst#1, in transmission 25. The MS cannot properly decode the data as indicated by the CRC check fail 26. Because the MS may perform soft combining with additional channel encoding information communicated later by the BS in accordance with the HARQ technique employed, the MS retains information from the first transmission 25 in a soft combining buffer in the MS, and returns a NACK 27. The BS receives the NACK and retransmits sub-burst#1 as illustrated. Cross-hatched box 28 represents the sub-burst#1 information present in the soft combining buffer in the receiver. The soft combining buffer has a fixed amount of memory space for storing information. This is indicated in FIG. 4 by the label “SOFT BUFFER SIZE”. After three transmissions 25, 29 and 30, the fourth transmission 31 of sub-burst#1 is not received by the MS as indicated by star 32. The MS does not receive fourth transmission 31, and therefore continues to wait for the fourth transmission of the sub-burst#1. The MS therefore continues to retain the sub-burst#1 information 28 in its soft combining buffer for soft combining with sub-burst#1 information in an upcoming fourth-transmission. From the BS perspective, however, the fourth transmission of sub-burst#1 failed. The BS therefore gives up communicating sub-burst#1 and transmits sub-burst#2 in transmission 32. The MS receives sub-burst#2 transmission 33, but its CRC check fails. The MS therefore should store the sub-burst#2 information 34 in the soft combining buffer for future soft combining of subsequent sub-burst#2 retransmissions. The sub-burst#2 information 34 pushed onto the soft combining buffer is indicated in FIG. 4 by hatched box 34. Because the sub-burst#1 information 28 from the failed sub-burst#1 communication is still occupying space in the soft combining buffer, the buffer may overflow when the sub-burst#2 information 34 is stored into the buffer as illustrated. Some or all of the sub-burst#2 information to be stored will be lost, resulting in degraded future performance of the HARQ system.
To address these and other problems, timers are sometimes employed in HARQ systems. FIG. 5 (Prior Art) is a diagram that illustrates operation of a conventional MS timer in a first situation. The system is a type of system referred to as an “asynchronous retransmission” system in that the time intervals between transmission and retransmission attempts by the BS may be of different durations. There may be a relatively irregular time separations between retransmission attempts in such an “asynchronous retransmission” system. FIG. 5 illustrates a scenario involving relatively short separation times whereas FIG. 6 illustrates a scenario involving relatively long separation times. Operation of the timer is represented by the line 35, the bowtie symbol 36, and the left-pointing arrow 37. Line 35 indicates that the timer starts timing upon receipt by the MS of a first transmission 38 of sub-burst#1. Because sub-burst#1 cannot be properly decoded as indicated by the CRC check fail (CRC NOK) 39, the timer starts timing an interval. If sub-burst#1 is not properly decoded before this interval elapses, then the MS is to flush the sub-burst#1 information from its soft combining buffer in an attempt to solve a potential buffer overflow problem. In the situation of FIG. 5, however, the timer interval is so long that the sub-burst#3 transmission 40 occurs before the timer expires. The previously described twice-toggling problem therefore occurs in this situation. The undecodable transmission 40 of sub-burst#3 is received, the MS detects the AI_SN sequence number to match the AI_SN sequence number of the transmissions of sub-burst#1, and the MS incorrectly attempts to soft combine transmission 40 with information stored in its soft combining buffer for sub-burst#1. The BS must retransmit sub-burst#3 in a later transmission 41 in order for the MS to receive the sub-burst. This is undesirable.
FIG. 6 (Prior Art) is a diagram that illustrates operation of a conventional MS timer in an asynchronous HARQ retransmission system in a second situation. In the situation illustrated in FIG. 6, the timer is set to expire in time interval 42 if sub-burst#1 has not been properly decoded by time 43. In the illustrated situation, the second retransmission 44 does not reach the MS. The timer expires at time 43, and the MS flushes information stored for sub-burst#1 from its soft combining buffer. After this flushing, the BS later attempts to retransmit sub-burst#1 in transmission 45. Soft combining in the MS, however, fails because the MS had previously flushed its soft combining buffer of sub-burst#1 information. In a worst case scenario, transmission 45 is the fourth transmission of sub-burst#1, and the BS gives up attempting to communicate sub-burst#1. Accordingly, in the situation of FIG. 5 the MS timer expired undesirably late, whereas in the situation of FIG. 6 the MS timer expired undesirably early.